In the past, mobile communication theory and practice have emphasized constant-envelope modulation schemes such as FM or CPM (Continuous Phase Modulation) in order to allow power amplifiers to work in the nonlinear operating zone near saturation. This permits the efficiency to be increased and do not generate intermodulation products in the adjacent channels.
At present, the main problem is due to limited portion available spectrum and therefore other types of modulation, spectrally more effective, have been taken into account, e.g. n-QPSK and, more in general, n-QAM with baseband filtering. Unfortunately these methods do not feature a constant envelope and the operating point of the RF power amplifier must be chosen with care to avoid disturbances on the spectrum due to intermodulation products. On the other hand, making an amplifier to work in the linear zone (class A) leads to a significant decrease in the efficiency and therefore suitable linearization techniques are to be adopted.
The idea of solving the distortion problem, caused by power amplifiers, through adaptive baseband-operated linearizing devices is not new.
In particular, the article "Adaptive Linearization using Predistortion" by M. Faulkner, T. Mattson and W. Yates IEEE CH 2846-4/90 pages 35 to 40, describes a predistortion device based upon the updating of error tables, whose content is used for correcting the baseband samples. In this case, an access mode to such tables is used which is directly proportional, in a uniform manner, to the amplitude of the input signal and to the distorted signal of the amplifier.
The main drawbacks encountered with this type of predistortion device, and also with others already known, are caused by slow convergence, inaccurate calculations, long processing times and loop instability phenomena.